Development process for permanently fixing an image on an electrophotographic copying material

ABSTRACT

A PROCESS FOR DEVELOPING AN ELECTROPHOTOGRAPHIC COPYING MATERIAL COMPRISING THE STEPS OF: DIPPING AN ELECTROPHOTOGRAPHIC COPYING MATERIAL IN A LIQUID DEVELOPER CONSISTING OF A CARRIER LIQUID AND A TONER DISPERSED THEREIN, SAID COPYING MATERIAL HAVING AN ELECTROSTATIC LATENT IMAGE ON A PHOTOCONDUCTIVE LAYER FORMED ON ONE SURFACE OF A SUPPORT WITH A BINDER, SAID DEVELOPER BEING PREPARED BY DISPERSING, IN SAID CARRIER LIQUID, A KNEADED MIXTURE CONSISTING OF PIGMENT PARTICLES, A RESINOUS POLARITY REGULATING AGENT AND A DYE INSOLUBLE IN SAID CARRIER LIQUID, WETTING A SURFACE OF A VISIBLE TONER IMAGE FORMED ON SAID PHOTOCONDUCTIVE LAYER WITH A SOLVENT CAPABLE OF DISSOLVING SAID DYE, AND THEN DRYING THE RESULTANT WETTED COPYING MATERIAL TO FIX A VISIBLE IMAGE CONSISTING OF SAID DYE IN SAID PHOTOCONDUCTIVE LAYER AND SAID SUPPORT SHEET.

United States Patent G F US. Cl. 117-37 6 Claims ABSTRACT OF THEDISCLOSURE A process for developing an electrophotographic copyingmaterial comprising the steps of:

dipping an electrophotographic copying material in a liquid developerconsisting of a carrier liquid and a toner dispersed therein, saidcopying material having an electrostatic latent image on aphotoconductive layer formed on one surface of a support with a binder,said developer being prepared by dispersing, in said carrier liquid, akneaded mixture consisting of pigment particles, a resinous polarityregulating agent and a dye insoluble in said carrier liquid,

wetting a surface of a visible toner image formed on saidphotoconductive layer with a solvent capable of dissolving said dye,

and then drying the resultant wetted copying material to fix a visibleimage consisting of said dye in said photoconductive layer and saidsupport sheet.

BACKGROUND OF THE INVENTION (a) Field of invention This inventionrelates to a process of developing electrophotographic copyingmaterials, and more particularly to a process of forming a lastingelectrophotographic print image which is not easily erasable when it isrubbed with a rubber eraser.

(b) Description of the prior art In the conventional electrophotographiccopying process using a liquid developer, the print image is normally nomore than a mere adsorption of developer particles (toner) consisting ofa pigment (normally carbon black is used) and resins to the surface of aphotoconductive layer of the copying material, or more precisely, to anelectrostatic latent image area formed on such photoconductive layer.Hence, the print image is easily erasable in most cases by rubbing itstrongly with a finger tip or a rubber eraser. The image quality ofelectrophotographic prints deteriorates with repeated usage of theprints. The same phenomenon is observed in an office Where the papersare circulated to many people.

SUMMARY OF THE INVENTION This invention provides a developing processfor forming a visible image not only on the surface of a photo-3,558,342 Patented Jan. 26, 1971 conductive layer of anelectrophotographic copying material, but also within thephotoconductive layer as well as on the support sheet thereof. In otherwords, the invention provides an improved electrophotographic developingprocess whereby the aforesaid drawback of the conventional developingprocess can be eliminated.

A suitable electrophotographic material for use in the developingprocess according to this invention will comprise a support sheet suchas paper, a plastic film or a metal sheet with powdered photoconductivematerial such as zinc oxide, titanium oxide, cadmium sulphide, leadsulphide and zinc sulphide, bound by a resinous binder to the surface ofsaid support sheet to form a photoconductive layer thereon. The presentinvention provides a process for forming, on a commercially availableelectrophotographic copying material, a copy image both on the surfaceof the photoconductive layer and also within said photoconductive layer.The process immediately preceding the formation of a copy image, namely,the process for providing an electrostatic latent image on the surfaceof a photoconductive layer, can be any of the known processes. Thegenerally known process for providing an electrostatic latent imagefirst provides the copying material with electrostatic photosensitivityby applying an electrostatic charge through corona discharge to thesurface of the photoconductive layer of said copying material, thenoverlaying same on an original form having the desired image to bereproduced on the copying material and then exposing same to whitelight.

The developer to be used for this invention comprises a carrier liquidand a toner dispersed therein, said toner consisting of a kneadedmixture of a dye insoluble in said carrier liquid, a carbon black and apolarity regulating resin or oil fat generally used for conventionaltoners. According to this invention, the steps of making the copy imageon the surface of a photoconductive layer of a copying material usingthe above-mentioned developer are not different from the conventionallyknown steps. Then, however, the surface of the copy image is wetted witha solvent which is capable of dissolving the aforesaid dye contained inthe toner. By this treatment, the dye dissolves out of the toner whichhas been adsorbed to the surface of the photoconductive layer and itpenetrates into the photoconductive layer, thereby forming, (within saidlayer, an image identical to the one formed on the surface by theadsorption of the toner. It has been observed that the image furtherpenetrates down to the support sheet when the latter is made of paper.Thus, inasmuch as the copy image is formed not only on the surface ofthe photoconductive layer of the copying material, but also within thelayer, the copy image within the layer remains unerased while the imageon the surface may be erasable.

Generally, zinc oxide powder used in a photoconductive layer makes thelatter relatively porous, and together with the paper base, the copyimage may be formed in threefold when development is made according tothis invention; namely, the image formed by the toner adsorption to thephotoconductive layer, the image formed by the dye penetration into saidlayer, and the image formed within the support sheet by said dye whichhas penetrated through the photoconductive layer.

The materials usable for a developer according to this invention aredescribed hereinbelow in further detail.

Materials for toner (a) Pigment particles:

Carbon black, indigo blue, rouge, phthalocyanine blue, (b) Dye:

Spirit black, aniline black, oil brown BB, oil blue G,

extra, oil red B, (c) Resinous polarity regulating agent:

Natural rosin, asphalt, alkyd resin compound, laurylmethacrylatedimethylaminoethylmethacrylate copolymer, and other synthetic resins.

Naturally, the resins usable for this invention are not limited to theabove-mentioned resins, but any resins known as polarity regulatingagents for liquid developing electrophotography can be used.

A developer according to this invention may be obtainable by making aconcentrated toner by kneading one of each of the aforesaid materials(a), (b) and (c) (preferably at the ratio of 1 part by weight of pigmentagainst 0.1-1 part by weight of dye and 0.5 parts by weight of resin(s))with a small amount of a kneading medium (normally, the same kind as thecarrier liquid is desirable) and then dispersing the resultantconcentrated toner in one of the following carrier liquids.

((1) Carrier liquid:

Isoparafl'inic hydrocarbons having an electric resistance of not lessthan 10 9 cm. and a dielectric constant of a maximum 3, normalparaffinic hydrocarbons and various petroleum hydrocarbons having thesame characteristics.

These hydrocarbons are sold by Esso Standard Oil Co.

under the trade names of Isopar-H and or Isopar-G.

A suitable solution for wetting the image surface after the image isformed by the aforementioned developer on the photoconductive layersurface will include: various kinds of alcohols (such as mono-valentalcohol: methylalcohol, ethylalcohol, propylalcohol, butylalcohol,polyvalent alcohol: ethyleneglycol, propyleneglycol, diethyleneglycol),various kinds of ketones (such as acetone, methylethylketone,methylisobutylketone, diethylketone), various kinds of ethers (such asethylether, isopropylether, 1,4-dioxane) and fatty acids (such as oleicacid, linolic acid, linolenic acid, elaeostearic acid).

These solvents can be directly sprayed or applied on the copy image, orsprayed with other solvents or mixed with emulsion. The desired resultcan be obtained so long as the surface of the copy image is wetted withthese solvents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Parts by wt. Carbonblack 0.5 Spirit black 1.0 High diesel HD 1 A concentrated toner wasmade by kneading the above mixture for hours in a ball mill, of which 4parts by weight were then dispersed in 1000 parts by weight of a carrierliquid of isoparaffinic hydrocarbon (Isopar-H) to make a developer forelectrophotographic use. In this developer was dipped, in order toproduce a print image, a sheet of commercially availableelectrophotographic copying sheet (Electronic Ricopy paper BS 1 made byKabushiki Kaisha Ricoh) carrying an electrostatic latent image processedby a normal electrophotographic method. The image surface was thenwetted with diethyleneglycol and the copying sheet was dried thereafter.

By rubbing the print image thus obtained with a rubber eraser, the imageportion formed by the toner adsorbed to the photoconductive layersurface disappeared, but the print image formed within thephotoconductive layer and the base material remained unerased.

Furthermore, the same results Were obtained when diethyleneglycol, or a50% solution thereof in water or in ethanol, was directly distributedon, or applied to an absorbent cotton for wetting the surface of theprint image processed in a developer comprising 1 part by weight ofcarbon black and 0.5 part by weight of spirit black but otherwise thesame composition as the previously mentioned developer.

Example 2 By applying acetic acid independently, or a solution thereofin water, onto the surface of a print image processed in the normalelectrophotographic method and developed with the same developer asdescribed in Example 1, and then drying the resultant sample, anunerasable print image was obtained.

Example 3 An unerasable print image was also obtained when the sameprocess as described in Example 1 was repeated using a mixture ofdiethyleneglycol and acetone mixed at the ratio of 3 parts by weight to1 part by weight.

Example 4 The same result was also obtainable by repeating the sameprocess as described in Example 1 using a mixture of diethyleneglycoland Inspregnol A Z (a wax emulsion made by Dainippon Ink and ChemicalsInc.) mixed at a ratio of 1 part by weight to 3 parts by weight.

What is claimed is:

1. A development process for permanently fixing an image on anelectrophotographic copying material having on one surface of a supporta photoconductive layer comprised of a resinous binder containingdispersed therein a powdered photoconductive material, said processcomprising the steps of:

(a) applying a liquid developer on said electrophotographic copyingmaterial having an electrostatic latent image on the surface of thephotoconductive layer thereof to form on said surface of saidphotoconductive layer a visible image corresponding to said latentimage, said developer consisting essentially of a carrier liquid and atoner uniformly dispersed therein, said toner being a uniform mixtureconsisting essentially of pigment particles, a dye insoluble in saidcarrier liquid but soluble in the followingnamed solvents and a resinouspolarity regulating agent selected from the group consisting of rosin,asphalt, alkyd resin andlaurylmethacrylate-dimethylaminoethylmethacrylate copolymer;

(b) then applying on the surface of said photoconductive layer carryingsaid visible image at least one liquid solvent for said dye, saidsolvent being selected from the group consisting of alcohols, ketones,ethers and oliphatic carboxylic acids, to form a liquid solution of saiddye so that said dye solution penetrates into and through thephotoconductive layer; and then (c) drying the thus treated copyingmaterial whereby the dye forms within said photoconductive layer avisible image corresponding to the visible image formed on the surfaceof said photoconductive layer.

2. A development process according to claim 1, wherein said toner is amixture consisting of carbon black, spirit black and said resinouspolarity regulating agent, and said solvent is diethyleneglycol.

3. A development process according to claim 1, wherein said toner is amixture consisting of 1 part by weight of said pigment particles, 0.1 to1 part by weight of said dye and 0.5 to 10 parts by weight of saidresinous polarity regulating agent.

4. A development process according to claim 1, wherein said pigment isselected from the group consisting of carbon black, indigo blue, rougeand phthalocyanine blue, and said dye is selected from the groupconsisting of spirit black, aniline black, oil brown BB, oil blue G.extra and oil red B.

References Cited UNITED STATES PATENTS Metcalfe et a1 117-33 Dessaver eta1 96-1 Walkup 101-1492 Carlson 34-77 6 Carlson 34-155 Carlson 117-21XOkishima 95-1.7 Brynko 96-1.4 Fauser 117-37 WILLIAM D. MARTIN, PrimaryExaminer E. J. CABIC, Assistant Examiner US. Cl. X.R.

